m4: fix -Wstrict-protoypes

In preparation for C23. Fixes the following error if Clang 16 is
cranked up to be stricter:
```
-warning: a function declaration without a prototype is deprecated in all versions of C [-Wstrict-prototypes]
+error: a function declaration without a prototype is deprecated in all versions of C [-Werror,-Wstrict-prototypes]
```

Bug: https://bugs.gentoo.org/879707


Signed-off-by: Sam James <sam@gentoo.org>